1. Field of the Invention
The present invention relates to laser amplification and optical switching systems, including but not limited to Pockels cells used for controlling light in lasers, optical switches, and other applications.
2. Description of Related Art
Electro-optical materials operate to change the polarization of a light beam in response to the application of an electrical voltage across the material. These materials are often used in combination with polarizers as electro-optical switches. In lasers or other optical systems, electro-optical materials are often configured as Pockels cells. Depending on the type and geometry of electro-optical material and the level of applied voltage, the polarization of a light beam can be varied to selectively pass a polarizer which has a predetermined polarization orientation. Thus, the transmission of a beam can be controlled as desired by application of a voltage.
Various different configurations of Pockels cell are known in the art for amplification of laser light, optical switching, and other applications. In one application for laser amplification, a laser medium in a regenerative amplifier cavity is pumped to generate an excess of excited atoms in the medium. A Pockels cell is then activated to capture a seed pulse in the cavity. The seed pulse is amplified by repeatedly passing through the laser medium. After a period of time, voltage is removed from the Pockels cell, thereby changing its polarization and causing the amplified pulse to be emitted from the cavity.
Dual-crystal Pockels cells with thermal compensation based on transverse effect are known in the art for providing optical switching using reduced control voltages. The dual-crystal Pockels cell uses two crystals in series, which reduces the magnitude of the applied voltage required to activate the cell. These are usually biaxial crystals and compensation is made for natural birefringence, which usually has a strong thermal dependence, by specially orienting the two crystals such that the beam passes along the X axis (for X-cut crystals) or the Y axis (for Y-cut crystals). Input beam polarization is directed at 45° with respect to the Y and Z axes, or alternatively the X and Z axes, depending on the crystal cut. The second crystal is rotated so that the Z axes of the two crystals sit at 90° relative to each other. The two crystals are also generally polished together to have matched lengths. The remainder of the discussion below assumes use of biaxial Y-cut crystals, with the understanding that the entire discussion equally applies to biaxial X-cut crystals.
The two crystals share a common central electrical contact. Voltage is applied between the common center electrode and the end electrodes, resulting in additive polarization change. Pockels cells of dual-crystal design have proven useful for Q-switched lasers where pulses in the range of 1 ns to 1000 ns are commonly generated, and for regenerative amplifiers in lasers where pulses in the range of 5 picoseconds to 1000 picoseconds are commonly generated. Short pulse widths in the range of about 30 femtoseconds to about 5 picoseconds are desirable for many applications, such as surgery or micro-machining, to precisely ablate targeted areas without damaging surrounding material.
Dual crystal Pockels cells are customarily constructed so as to ensure that their crystal structures are aligned. The pitch and yaw of the two crystals, i.e., their rotational orientation for Y-cut crystals with respect to the Z and X axes, respectively, is controlled using mounting fixtures to ensure that the Y axes of the two crystals are parallel. The extinction ratio of the dual-crystal Pockels cell depends on the precision with which the Y axes of the two crystals are parallel. The dual crystals are customarily factory-installed in a structure so as to achieve the desired axial alignment, and locked into position. Normally, this alignment is fixed and not adjustable. The amount of precision in this alignment, however, is directly related to the extinction ratio of the emitted pulse.
In addition, the crystals in a dual-crystal Pockels cell are rotated around their Y axes with respect to one another to achieve thermal compensation. The amount of rotation is nominally 90°, such that one crystal of the pair is rotated around its Y axis by this amount, relative to the Z (or X) axis of the other crystal. Again, the crystals are normally factory-installed with this rotational offset, and it too is not normally adjusted during operation of the Pockels cell. The precision of this rotational offset is generally about ±60 minutes.
Notwithstanding the advantages of dual crystal Pockels cells and systems that employ them, these systems are subject to certain disadvantages, notably when used in regenerative amplifiers for amplification of laser pulses of about 30 femtosecond to about 5 picoseconds. Laser pulses from high repetition rate lasers tend to occur with sidebands, which reduces the peak laser intensity. These sidebands may not be observable for slower pulses, for example, pulses in the 5 picosecond or longer range. However, such sidebands become readily apparent for laser pulses on the order of picoseconds or shorter. Control of these sidebands, whether to increase the intensity of the peak pulse or for more control over the shape of the peak pulse and the resulting sidebands, is desirable.